The present invention relates to systems for transmitting information from a plurality of seismic data acquisition means to a central receiver and recording means, comprising a plurality of transmission modules each connected to the data acquisition means.
Seismic data acquisition means are usually made up of a group of hydrophones (or geophones) which provide data in the form of analog signals. If each seismic data acquisition means has M channels and N such data acquisition means are used in one array, M .multidot. N data channels are required for data transmission. The transmission distance is typically from 3 to 5 km.
For practical reasons, seismic cables are made in sections of 50 to 100 meters in length with electrical connectors at each end to permit the cables to be connected in series to obtain the desired length and number of channels. The conventional method is to utilize twisted pairs for analog transmission of the seismic data, using one pair for each channel. The data are digitized in the central receiver and recording means and stored on magnetic tape for subsequent processing in the computer.
A major weakness of such conventional methods is that due to the very large number of electrical contacts in the connectors, the probability of faults is high. A further drawback is that due to the large number of electrical conductors, the cable becomes large, heavy and difficult to handle. This, in practice, imposes an upper limit on the number of transmission channels possible, a limit of 240 channels being the maximum. The conventional method of transmitting data in the form of analog signals also results in a reduction in the quality of the data during transmission to the central receiver and recording means.
It has been recognized in the prior art that the above drawbacks mentioned can be reduced significantly by digitizing the seismic data in units located in the immediate vicinity of the acquisition means and transmitting the digital data on a digital transmission system to the central receiver and recording means. However, a new and important drawback arises with such a system, in that complicated electronics components must be distributed throughout the system. This is especially a problem in connection with marine seismic data acquisition since several hours of expensive ship time are required to wind the cable in and out if a defective module has to be replaced. Although several different brands of digital transmission systems for marine seismic data acquisition, so-called digital streamers are commercially available. All of these systems have the weakness that they are not constructed to function satisfactorily if faults occur either in the transmission lines or in the electronics part of the system.
The requirements which must be met by digital transmission systems for seismic streamers differ in certain fundamental areas from other known data transmission systems. A characteristic feature of general digital transmission in a seismic streamer is the exceedingly critical balance that must be struck between the requirements of reliability and complexity, power consumption, weight and volume. All of the electronics must be placed inside water-tight housings between the sections of seismic cable. These housings must be as short as possible to allow the cable to be wound up on a winch, and they must also be slim in order to avoid any turbulence which would generate acoustic noise when the housings are towed through the water. The power consumption of a digital seismic streamer can easily be 1 kW or more. Over the transmission distances in question, and with the voltages that are permitted to be used, the conductor cross section becomes large, resulting in a heavy and cumbersome cable. The weight of both the cable and the electronics housing is a very important factor, since the cable of necessity must have the same specific gravity as the water, or, in other words, be neutrally buoyant.
In view of the above-mentioned drawbacks associated with digital data transmission systems, it would be desirable to have complete redundancy by providing duplicate sets of the electronics units. However, for the reasons mentioned above, it has not been possible to provide duplicate sets of electronics seismic streamers. With the present invention, however, a total solution is provided which, in a simple manner, provide essentially the same advantages as a complete double set of electronics, with complete switching capability in each module to the backup of electronics and to the spare line. At the same time the system has a modular construction which makes it particularly well suited for implementation with customer-specified integrated circuits, thereby providing important advantages with respect to weight and volume.